Avaya 1000 Series Configuration Manual page 212

OSPF Routing Protocol
to the same physical media device and, by nature, it is multi-access. Some examples of this
are Frame Relay networks, ATM networks and x.25 networks.
To achieve this functionality, some components of OSPF have been modified in an attempt to
mirror functionality found in OSPF broadcast networks. Two modes of operation on these types
of OSPF networks are NBMA and P2MP. When using NBMA, operation over a broadcast
network is emulated by OSPF. The NBMA network has a router designated to originate a
network LSA. NBMA mode is the most efficient way to run OSPF over non-broadcast networks,
both in terms of link-state database size and in terms of the amount of routing protocol traffic.
When deploying OSPF on a network, neighbor discovery is achieved using multicast hello
packets. Designated Routers (DR) and Backup Designated Routers (BDR) are elected for each
multicast network in order to optimize adjacency building. All routers in a segment should
communicate directly with a DR or BDR for proper adjacency. For a neighbor to be successfully
discovered on a segment, broadcast and multicast packet sending must be allowed on the
network.
When using NBMA technology, neighbors are not discovered automatically due to the non-
broadcast nature of the feature. Instead, OSPF attempts to designate a DR and a BDR, but
the election fails since no neighbors are discovered. In order to overcome this issue, neighbors
must be manually configured.
Broadcast vs non-broadcast networks
One difference between broadcast and non-broadcast networks is in the functionality of the
hello protocol. On a broadcast network, a router advertises itself using hello packets allowing
itself to be discovered dynamically. These packets include the router's DR identity and a list
of routers who have recently send Hello packets. On NBMA networks, some configuration must
take place before successful operation of the hello protocol. Routers that are potential DRs
have a list of all other routers currently attached. If a DR candidate, a router sends Hello packets
to other candidates in an attempt to find a DR. If elected DR, a router sends hello packets to
all other routers on the network. To minimize the number of hello packets sent, the number of
eligible routers on a NBMA network should be kept to a minimum.
The behavior of any router's hello packet sending depends on its status as potential DR. If
eligible, it must send hello packets to eligible neighbors periodically. If the router becomes the
DR or BDR, it expands distribution of hello packets to include all neighbors, regardless of
eligibility. If a router is not eligible, it must send hello packets to the DR and BDR periodically,
along with sending a reply hello packet to any hello packet received from an eligible neighbor.
Frequency of hello packets depends on a neighbor's state. When down, hello packets are sent
at Poll Interval, otherwise they are sent at Hello Interval.
Another difference comes when identifying a neighbor address. In a point-to-point network or
virtual link, the neighbor is identified by router ID. However, in a broadcast, point-to-
multipoint or NBMA network, the neighbor is identified by IP source address.
Finally, in an OSPF operation specific to NBMA, OSPF generates a start event to a neighbor
when the neighbor command is issued. When this occurs, hello packets begin to be sent to a
neighbor using the Hello Interval as a frequency. This causes the neighbor to receive an
ATTEMPT message that indicates no recent information has been received from the neighbor
and that a greater effort is to be to contact that neighbor. To achieve this, up to four hello packets
212 Avaya Secure Router 1000 Series Configuration Guide December 2010